1. Field of the Invention
The present invention relates to a hydrostatic transaxle, which supports a pair of left and right steerable drive wheels, and incorporates a pair of hydraulic motors for driving the respective drive wheels. Further, the present invention relates to a hydraulically driven vehicle, which comprises a common hydraulic pressure source and at least two hydraulic motors connected in series to the common hydraulic pressure source, wherein one of the at least two hydraulic motors drives front drive wheels (or a front drive wheel), and the other drives rear drive wheels (or a rear drive wheel).
2. Related Art
Conventionally, as disclosed in International Publication WO 2004/062956, there is a well-known hydraulically driven vehicle having a main transaxle and an auxiliary transaxle. The main transaxle supports main drive wheels, and comprises a hydraulic motor for driving the main drive wheels. The auxiliary transaxle supports a pair of auxiliary drive wheels, and comprises a pair of hydraulic motors for driving the respective auxiliary drive wheels. The vehicle has a hydraulic circuit fluidly connecting the hydraulic motors of the main and auxiliary transaxles to a hydraulic pump. In the hydraulic circuit, the hydraulic motor of the main transaxle and the pair of hydraulic motors of the auxiliary transaxle are connected in series to the hydraulic pump, and the hydraulic motors of the auxiliary transaxle are connected to the hydraulic pump in parallel to each other.
The left and right auxiliary drive wheels are steerable wheels. Both of the hydraulic motors of the auxiliary transaxle are variable in displacement so as to prevent drag of the steerable wheels or the main drive wheels. The hydraulic motors of the auxiliary transaxle are provided with respective movable swash plates moved according to steering operation of the vehicle, or according to change of left or right turning angle of the steerable wheels, so as to change rotary speed of the steerable wheels, i.e., generate difference between the main drive wheels and the auxiliary drive wheels, according to change of the left or right turning angle of the vehicle. However, the link mechanism for moving the two movable swash plates according to the turning of the steerable wheels is complicated, and expanded so as to reduce spaces for other parts. Further, the link mechanism requires a large operation force because of its small operational efficiency.
Further, the conventional vehicle is set so that peripheral speed of the front drive wheels becomes equal to peripheral speed of the rear drive wheels during straight traveling. However, there is a problem that the peripheral speed of the drive wheel may be unexpectedly reduced because of bad road condition so as to be slower than that corresponding to the amount of fluid flowing through the corresponding hydraulic motor, thereby causing stiff traveling of the vehicle.
Further, with respect to the conventional hydraulically driven vehicle, as disclosed in International Publication WO 2004/062956, the main transaxle is expanded because it incorporates the hydraulic pump together with the hydraulic motor for driving the main drive wheels. Such a large-size transaxle may reduce variation of adaptable vehicles in size. Further, a pump shaft of the hydraulic pump projects outward from the transaxle so as to be provided thereon with a cooling fan. The expansion of the transaxle means that the area of the transaxle blown by cooling wind generated from the cooling fan becomes small relative to the transaxle. Besides, to efficiently cool fluid circulating in the HST fluid circuit having the series connection of the hydraulic motor for the main drive wheels and the pair of hydraulic motors for the auxiliary drive wheels to the hydraulic pump, it is advantageous that pipes for circulating fluid are blown by cooling wind generated from the cooling fan. However, it is difficult to concentrating the pipes adjacent to the cooling fan of the large-size transaxle. In this way, the conventional combination of the main transaxle incorporating the hydraulic pump and the hydraulic motor for driving the main drive wheels with the auxiliary transaxle incorporating the pair of hydraulic motors for driving the auxiliary drive wheels cannot ensure sufficient cooling efficiency for the HST fluid circuit.